The present invention relates to an induction heating circuit for cooker, e.g., a rice cooker. The rice cooker has a housing enclosing a metal bowl, a heating coil, and a ceramic provided between the heating coil and the metal bowl.
There are several of circuit used to apply high frequency AC voltage on the heating coil. Half bridge converter type circuit and push-pull converter type circuit use two power transistors as switches. Class-E converter type circuit use only one power transistor as a switch. For lower manufacturing cost, Class-E converter type is used more widely than half-bridge converter type. The push-pull converter type is not widely used since it requires complicated heating coil. However, the push-pull converter type circuit is better suited for applying power to the heating coils than the class-E converter type.
The heating bowls are made generally using double layers of metal. The inner layer is aluminum and the outer layer is iron. The outer layer of iron is to increase the resistance of the bowl. Without the outer iron layer, the resistance of the bowl may be too low and the class-E converter type circuit may not operate properly. However, the push-pull converter type circuit can operate with a bowl having a low resistance.
The push-pull converter type circuit requires a heating coil to have a center-tap to operate properly. The inductance of the two parts of the coil should be substantially the same for proper operation of the push-pull converter type circuit. For this purposes, the two parts of the coil, i.e., the first part running from the beginning of the coil to the center-tap and the second part from the center-tap to the ending of the coil, should be similar to each other in shape and geometry.